In early January I spotted this pair of lenticular clouds in the Bighorn Mountains, and I reached a lookout just in time to watch them light up after sunset. These lens-shaped clouds appear to remain stationary, and are fascinating to watch. When strong winds flow over a mountain range, the air gets deflected and standing waves are formed on the lee (downwind) side. If there is enough moisture in the air, then the crest of the wave condenses into a cloud. Mountain waves can cause severe turbulence in passing aircraft. Even though these waves may be present in clear air, lenticular clouds serve as a visual warning to pilots to stay away. UFO sightings have been attributed to these clouds as well, since many of them do resemble flying saucers.